With the increase in system performance, the challenges associated with the impact of higher frequencies on signal integrity have similarly increased. Transmission line effects can significantly affect transmitted data, particularly as transmission speeds increase. Other potentially problematic effects may include ringing, crosstalk, inter-symbol interference, simultaneous switching noise, power integrity, low immunity to power bouncing, and overshooting. Any one of these problems could seriously impact signal integrity, particularly for high-speed and high-throughput applications (e.g., DDR memory).
Impedance mismatch may be a culprit for a number of the foregoing problems. Impedance mismatch may be caused when the output impedance of the source, the impedance of the transmission line, and the impedance of the receiver are not equal.
Currently, the output impedance may be controlled in various ways including adding an output buffer, such as the complementary CMOS output buffer. Although it is possible to size the CMOS output buffer to have the desired output impedance, the output impedance generally depends on the applied gate-source voltage, Vgs. During commutation (signal switching), however, the CMOS transistor may have a variable impedance, resulting in the output impedance not matching the transmission line impedance. In fact, when Vgs=0V the output impedance is infinite, when Vgs=Vccq the output impedance is equal to the impedance Zo of the transmission line, and when Vgs is somewhere between 0V and Vccq (e.g., during the switching on phase) the output impedance is also somewhere between 0V and Vccq infinite and Zo but different than Zo (i.e., the impedance does not adapt).
The output buffer should also limit the current peak slope (di/dt) to limit the internal Vccq/Vssq noise produced by series inductance. To do this, the output edge slope is sometimes slowed down. Doing this, however, it takes longer to transit between infinite impedance and Zo value, resulting sometimes in inter-symbol interference effects.
Moreover, the typical CMOS output buffer generally transmits the supply noise directly onto the transmission line without attenuation. In fact, the resistance of the CMOS transistor may be equal to the transmission line impedance, and there may be a direct connection between the noisy supply and the output.